Recently, secondary batteries refer to devices which convert exterior electrical energy into chemical energy and store the same, and generate electricity as needed. The secondary batteries are also referred to as “rechargeable batteries” because they can be recharged repeatedly. Generally used secondary batteries include lead accumulators, nickel cadmium (NiCd) batteries, nickel hydride (NiMH) accumulators, lithium ion (Li-ion) batteries, lithium ion (Li-ion) polymer batteries, and the like. The secondary batteries are more economically and environmentally advantageous than disposable primary batteries.
Secondary batteries are currently used in applications requiring low electric power. Examples of the applications include equipment to aid to start vehicles, mobile devices, tools, uninterruptible power supplies and the like. The recent development of wireless communication technologies has brought about not only the popularization of mobile devices but also the mobilization of many kinds of conventional devices, and the demand for secondary batteries has been dramatically increasing. In addition, hybrid vehicles and electric vehicles have been commercialized for the purpose of preventing environmental pollution or the like and these next-generation vehicles utilize secondary batteries to reduce the costs and weight and to increase lifespan.
In general, most of secondary batteries have a cylindrical, prismatic, or pouch shape. This is because secondary batteries are produced by mounting an electrode assembly including a negative electrode, a positive electrode and a separation film in a cylindrical or prismatic metal can or a pouch-shaped case of an aluminum laminate sheet and incorporating an electrolyte into the electrode assembly. Because a predetermined space for mounting the secondary battery is necessary, the cylindrical, prismatic or pouch shape of the secondary batteries is disadvantageously a limitation in developing various shapes of mobile devices. Accordingly, there is a need for a new structure of secondary batteries which are easily changeable in shape.
In response to this need, cable-type batteries having a very high ratio of length to cross-sectional diameter have been suggested. However, when active materials used for cable-type batteries are coated above a predetermined thickness or load in order to increase a battery capacity per unit length of cable-type batteries, there are problems such as increased thickness of active material layers, thus increase in electrode resistances and thus deterioration of battery performances, lifespan, output characteristics and the like. Accordingly, there is a limitation in improving battery capacity.